The present invention generally relates to protection circuits and battery units, and more particularly to a protection circuit and a battery unit which prevent excessive discharge and excessive charging of a battery.
Recently, in portable electronic equipments typified by notebook type personal computers (or lap-top computers) and the like, lithium ion (Li.sup.+) batteries or the like are replacing nickel cadmium (NiCd) batteries, nickel metal hydrogen (NiMH) batteries and the like. Compared to the NiCd battery, the NiMH battery and the like, the Li.sup.+ battery is lighter and has a greater capacity per unit volume. Accordingly, the Li.sup.+ battery is suited for use in equipments which must satisfy demands such as light weight and continuous use for a long period of time.
In the battery unit which is used in the portable electronic equipment or the like, a plurality of battery cells are connected in series due to an output voltage that can be output from a single battery cell. A maximum number of battery cells that can be connected in series within the battery unit is determined by the relationship of an output voltage of the battery unit and a power supply voltage that is supplied from the outside when charging the battery unit. For example, the output voltage of one NiCd battery cell or a NiMH battery cell is 1.2 V, and the power supply voltage that is supplied when charging the battery unit is approximately 1.7 V. When a withstand voltage of parts of a power supply system of the general portable electronic equipment or the like, an input voltage of an A.C. adapter and the like are taken into consideration, the battery unit is most conveniently used when the output voltage of the battery unit is approximately 16.0 V, and in the case where the NiCd battery cells or the NiMH battery cells are used, the maximum number of battery cells that can be connected in series within the battery unit is nine. On the other hand, the output voltage of a single Li.sup.+ battery cell is approximately 4.2 V at the maximum. Accordingly, the maximum number of Li.sup.+ battery cells that can be connected in series within the battery unit is approximately three.
Unlike the NiCd battery unit or the NiMH battery unit, the Li.sup.+ battery unit is provided with a protection function against short-circuits outside the Li.sup.+ battery unit and short-circuits within the Li.sup.+ battery unit. Because the capacity of the Li.sup.+ battery unit per unit volume is large, energy is discharged within a short time if the output of the Li.sup.+ battery unit is short-circuited for some reason or a short-circuit occurs within the Li.sup.+ battery unit for some reason, and in such cases, there is a possibility of the Li.sup.+ battery unit becoming deteriorated or the serviceable life of the Li.sup.+ battery unit becoming shortened. Hence, the protection function is provided for this reason. Accordingly, even if a short-circuit occurs outside or inside the Li.sup.+ battery unit, an excessive discharge current or an excessive charging current is cut off by a fuse or the like when the charging current or the discharge current becomes greater than a predetermined value, thereby preventing deterioration of the Li.sup.+ battery unit and securing the serviceable life of the Li.sup.+ battery unit.
On the other hand, the capacity of each battery cell within the battery unit is determined by a basic capacity which is based on the size of the battery unit. Hence, in order to increase the capacity of the battery unit, it becomes necessary to connect a plurality of battery cells in parallel, and to connect such parallel connections in series.
FIG. 1 is a circuit diagram showing an example of a conventional battery unit, and FIG. 2 is a circuit diagram showing the construction of a voltage monitoring circuit within the battery unit shown in FIG. 1.
In FIG. 1, a battery unit 100 generally includes battery cells E11, E12, E21, E22, E31 and E32, a voltage monitoring circuit 101, a fuse 102, P-channel field effect transistors (FETs) 103 and 104, and power supply terminals 105 and 106 which are connected as shown. The battery cells E11 and E12 are connected in parallel, the battery cells E21 and E22 are connected in parallel, and the battery cells E31 and E32 are connected in parallel. In addition, the parallel connection of the battery cells E11 and E12, the parallel connection of the battery cells E21 and E22, and the parallel connection of the battery cells E31 and E32 are connected in series.
The voltage monitoring circuit 101 monitors the voltages of the parallel connection of the battery cells E11 and E12, the parallel connection of the battery cells E21 and E22, and the parallel connection of the battery cells E31 and E32, and detects an excessive discharge state in the discharging state of the battery unit 100 if the voltage of one of the parallel connections of the battery cells becomes less than a predetermined value. When the excessive discharge state is detected, the voltage monitoring circuit 101 turns OFF the FET 103 so as to cut off the discharge current from the battery unit 100 and to prevent the excessive discharge. On the other hand, the voltage monitoring circuit 101 monitors the voltages of the parallel connection of the battery cells E11 and E12, the parallel connection of the battery cells E21 and E22, and the parallel connection of the battery cells E31 and E32, and detects an excessive charging state in the charging state of the battery unit 100 if the voltage of one of the parallel connections of the battery cells becomes greater than a predetermined value. When the excessive charging state is detected, the voltage monitoring circuit 101 turns OFF the FET 104 so as to cut off the charging current to the battery unit 100 and to prevent the excessive charging.
The fuse 102 melts and breaks the connection when a current greater than a predetermined value flows through the fuse, so as to cut off the current flow. As a result, even if the operation of cutting off the excessive current by the voltage monitoring circuit 101 does not function correctly or the operation of cutting off the excessive current does not function correctly due to a failure such as short-circuiting of the FETs 103 and 104 themselves, the fuse 102 melts and breaks the connection to provide a double protection circuit.
The voltage monitoring circuit 101 includes comparator circuits 111 through 113 and 121 through 123, and logical sum (OR) circuits 114 and 124 which are connected as shown in FIG. 2. In FIG. 2, e1 and e2 respectively denote reference voltages indicating an excessive discharge limit voltage and an excessive charging limit voltage of the battery cells E11 through E32.
Generally, the conventional battery unit is made up of a single battery unit or a plurality of battery cells connected in series, and for this reason, no special consideration is given with respect to a case where the battery cells are connected in parallel. However, as the number of battery cells provided within the battery unit increases, although the possibility of a short-circuit occurring within the battery unit due to an abnormality generated in a battery cell is extremely small, the possibility is not zero. For this reason, when a plurality of battery cells are connected in parallel and such parallel connections are connected in series within the battery unit in order to increase the capacity of the battery unit, a current which flows through the battery cell in which the abnormality is generated becomes an integral multiple of that during a normal state, where the integral multiple corresponds to the number of battery cells connected in parallel within the parallel connection. As a result, there was a problem in that considerable deterioration and considerable shortening of the serviceable life occurs when the above described abnormality occurs in the Li.sup.+ battery unit having such parallel connections.
For example, in the case of the battery unit 100 shown in FIGS. 1 and 2, if an internal short-circuiting occurs in the battery cell E11 which is connected in parallel with the battery cell E12, the energy stored in the battery cell E11 is consumed instantaneously, and the current from the other battery cell E12, which is connected in parallel with the battery cell E11, also flows to the battery cell E11. In other words, a current which is an integral multiple (in this case, two times) of that during the normal state is instantaneously consumed by the battery cell E11, and there is a possibility that the battery unit 100 as a whole will deteriorate and the serviceable life of the battery unit 100 will become shortened.